Cryostat for magnetic resonance imaging system

ABSTRACT

Cryostat systems for magnetic resonance imaging system are provided. The cryostat system may include a tank containing a cavity to accommodate a cooling medium and a superconducting coil. The system may also include a cold head assembly configured to cool the cooling medium to maintain the superconducting coil in a superconducting state. The cold head assembly may be mounted on the tank. The cold assembly may include at least a first cold head and a second cold head. The second cold head may include a taper shape with a first end surface close to the first cold head and a second end surface away from the first cold head. A diameter of the first circular end is greater than a diameter of the second circular end.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims the benefits of priorityto Chinese Patent Application No. 201711295894.1, filed on Dec. 8, 2017,the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a medical imaging system, and moreparticularly to, a cryostat device of a magnetic resonance imagingsystem.

BACKGROUND

Magnetic resonance imaging (MRI) systems can obtain one-dimensional (1D)images, two-dimensional (2D) images, and three-dimensional (3D) imagesof a subject for diagnosis. MRI systems are often used to diagnosepathology and internal injuries. A typical MRI system may include asuperconducting coil generating a strong and uniform main magnetic fieldin a scanning area of the MRI system. The superconducting coils may needto work properly at an environment with an extremely low temperature.The superconducting coils may be placed in a tank containing cryogen asliquid helium. A refrigerator may be used to recondense the boiling gashelium to liquid, and provide the cooling capacity to the tank tomaintain the low temperature environment that the superconducting coilsneeds.

The refrigerator may provide the adequate cooling capacity to the tank,which may compensate a cooling capacity against heat leakage from anambient environment of the MRI system. The cryostat system may bemaintained in a stable low-temperature condition to ensure thesuperconducting coil to work properly. The refrigerator may include a GM(Gifford-McMahon) refrigerator and/or a pulsatron refrigerator.

However, a lifetime of the GM refrigerator may be about 10,000 hours,which means that it is necessary to perform a removal and/or aninstallation operation from/to the cold head sleeve in the cryostatsystem. In the hospital, a space between the cold head sleeve in thecryostat system and a ceiling above the cryostat system may be limited.It is therefore desirable to provide systems and methods for assemblingthe cold head sleeve in the cryostat system that may decrease the spacerequired between the cold head and the ceiling during the removal and/orinstallation operations to the cold head assembly.

SUMMARY

In one aspect of the present disclosure, a cryostat for a magneticresonance imaging system is provided. The system may include a tankcontaining a cavity to accommodate a cooling medium and asuperconducting coil, and a cold head assembly configured to cool thecooling medium to maintain the superconducting coil in a superconductingstate. The cold head assembly may be mounted on the tank. The centralaxis of the cold head assembly and a vertical line may form an angle.The vertical line may pass a reference point of the central axis of thecold head assembly. The vertical line may also be parallel to a verticalplane including an axis of the superconducting coil and a cross-sectionof the superconducting coil. The cold head assembly may include a firstcold head and a second cold head. The second cold head may have a tapershape with a first end surface close to the first cold head and a secondend surface away from the first cold head.

In some embodiments, the cold head assembly may be inclined awayrelative to the vertical plane.

In some embodiments, the cold head assembly may be inclined along theaxis of the superconducting coil.

In some embodiments, the angle may be equal to or less than 45 degrees.

In some embodiments, a surface of the second cold head may include aplurality of protrusions. A recess may be formed between two adjacentprotrusions of the plurality of protrusions.

In some embodiments, one of the plurality of protrusions may have afin-like shape. The one of the plurality of protrusions may have a firstend close to the first cold head and a second end away from the firstcold head. The thickness of the first end of the one of the plurality ofprotrusions may be greater than the thickness of the second end of theone of the plurality of protrusions.

In some embodiments, the surface of the second cold head or a surface ofthe plurality of protrusions may be polished or has a plating layer.

In some embodiments, the system may further include a radiation shieldand a vacuum layer. The radiation shield and the vacuum layer mayenclose the tank. The system may also include a heat conductive bandlocated in the vacuum layer and configured to thermal connect theradiation shield and the cold head assembly.

In some embodiments, the taper shape may include a shape of triangularprism, polygonous prism or truncated cone

In some embodiments, a diameter of the first end surface may be greaterthan a diameter of the second end surface.

In another aspect of the present disclosure, a system is provided. Thesystem may include a cold head assembly. The cold head assembly mayinclude at least a first cold head and a second cold head. The secondcold head may have a taper shape with a first end surface close to thefirst cold head and a second end surface away from the first cold head.The diameter of the first end surface may be greater than a diameter ofthe second end surface.

In some embodiments, a surface of the second cold head may include aplurality of protrusions, a recess being formed between two adjacentprotrusions of the plurality of protrusions.

In some embodiments, one of the plurality of protrusions may have afin-like shape. The one of the plurality of protrusions may have a firstend close to the first cold head and a second end away from the firstcold head. The thickness of the first end of the one of the plurality ofprotrusions may be greater than the thickness of the second end of theone of the plurality of protrusions.

In some embodiments, the surface of the second cold head or a surface ofthe plurality of protrusions may be polished or have a plating layer.

In some embodiments, the system may further include a tank containing acavity to accommodate a cooling medium and a superconducting coil, and acold head assembly configured to cool the cooling medium to maintain thesuperconducting coil in a superconducting state. The cold head assemblymay be mounted on the tank. The central axis of the cold head assemblyand a vertical line may form an angle. The vertical line may pass areference point of the central axis of the cold head assembly. Thevertical line may also be parallel to a vertical plane including an axisof the superconducting coil and a cross-section of the superconductingcoil.

In some embodiments, the cold head assembly may be inclined awayrelative to the vertical plane.

In some embodiments, the cold head assembly may be inclined along theaxis of the superconducting coil.

In some embodiments, the angle may be equal to or less than 45 degrees.

In some embodiments, the system may further include a radiation shieldand a vacuum layer. The radiation shield and the vacuum layer mayenclose the tank The system may also include a heat conductive bandlocated in the vacuum layer and configured to thermal connect theradiation shield and the cold head assembly.

In some embodiments, the taper shape may include a shape of triangularprism, polygonous prism or truncated cone.

Additional features will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the artupon examination of the following and the accompanying drawings or maybe learned by production or operation of the examples. The features ofthe present disclosure may be realized and attained by practice or useof various aspects of the methodologies, instrumentalities andcombinations set forth in the detailed examples discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is further described in terms of exemplaryembodiments. These exemplary embodiments are described in detail withreference to the drawings. These embodiments are non-limiting exemplaryembodiments, in which like reference numerals represent similarstructures throughout the several views of the drawings, and wherein:

FIG. 1 is a schematic diagram illustrating an exemplary cryostat systemaccording to some embodiments of the present disclosure;

FIG. 2 is a schematic diagram illustrating an exemplary cold headposition of the cryostat system 100 in hospital field according to someembodiments of the present disclosure;

FIG. 3 is a schematic diagram illustrating an exemplary cold headassembly according to some embodiments of the present disclosure;

FIG. 4 is a schematic diagram illustrating an exemplary second cold headaccording to some embodiments of the present disclosure;

FIG. 5 is a schematic diagram illustrating an exemplary second cold headaccording to some embodiments of the present disclosure;

FIG. 6 is a schematic diagram illustrating a part of an exemplary secondcold head according to some embodiments of the present disclosure; and

FIG. 7 is a schematic diagram illustrating an exemplary second cold headaccording to some embodiments of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant disclosure. However, it should be apparent to those skilledin the art that the present disclosure may be practiced without suchdetails. In other instances, well-known methods, procedures, systems,components, and/or circuitry have been described at a relativelyhigh-level, without detail, in order to avoid unnecessarily obscuringaspects of the present disclosure. Various modifications to thedisclosed embodiments will be readily apparent to those skilled in theart, and the general principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present disclosure. Thus, the present disclosure is not limitedto the embodiments shown, but to be accorded the widest scope consistentwith the claims.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise,”“comprises,” and/or “comprising,” “include,” “includes,” and/or“including,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

It will be understood that the term “system,” “unit,” “module,” and/or“block” used herein are one method to distinguish different components,elements, parts, section or assembly of different level in ascendingorder. However, the terms may be displaced by another expression if theyachieve the same purpose.

Generally, the word “module,” “unit,” or “block,” as used herein, refersto logic embodied in hardware or firmware, or to a collection ofsoftware instructions. A module, a unit, or a block described herein maybe implemented as software and/or hardware and may be stored in any typeof non-transitory computer-readable medium or other storage device. Insome embodiments, a software module/unit/block may be compiled andlinked into an executable program. It will be appreciated that softwaremodules can be callable from other modules/units/blocks or fromthemselves, and/or may be invoked in response to detected events orinterrupts. Software modules/units/blocks configured for execution oncomputing devices may be provided on a computer readable medium, such asa compact disc, a digital video disc, a flash drive, a magnetic disc, orany other tangible medium, or as a digital download (and can beoriginally stored in a compressed or installable format that needsinstallation, decompression, or decryption prior to execution). Suchsoftware code may be stored, partially or fully, on a storage device ofthe executing computing device, for execution by the computing device.Software instructions may be embedded in firmware, such as an EPROM. Itwill be further appreciated that hardware modules/units/blocks may beincluded of connected logic components, such as gates and flip-flops,and/or can be included of programmable units, such as programmable gatearrays or processors. The modules/units/blocks or computing devicefunctionality described herein may be implemented as softwaremodules/units/blocks, but may be represented in hardware or firmware. Ingeneral, the modules/units/blocks described herein refer to logicalmodules/units/blocks that may be combined with othermodules/units/blocks or divided into sub-modules/sub-units/sub-blocksdespite their physical organization or storage.

It will be understood that when a unit, engine, module or block isreferred to as being “on,” “connected to,” or “coupled to,” anotherunit, engine, module, or block, it may be directly on, connected orcoupled to, or communicate with the other unit, engine, module, orblock, or an intervening unit, engine, module, or block may be present,unless the context clearly indicates otherwise. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items.

These and other features, and characteristics of the present disclosure,as well as the methods of operation and functions of the relatedelements of structure and the combination of parts and economies ofmanufacture, may become more apparent upon consideration of thefollowing description with reference to the accompanying drawings, allof which form a part of this disclosure. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended to limit thescope of the present disclosure. It is understood that the drawings arenot to scale.

The following descriptions are provided with reference to a coolingtechnique for maintaining a superconducting coil of an imaging system ina superconducting state. The imaging system may be used for non-invasiveimaging, which may be used for disease diagnosis, disease treatment orresearch purposes. In some embodiments, the imaging system may includeone or more modalities including Magnetic Resonance Imaging (MRI),Magnetic Resonance Angiography (MRA), CT (computed tomography)-MR, DSA(digital subtraction angiography)-MR, PET (positron emissiontomography)-MR, TMS (transcranial magnetic stimulation)-MR, US(ultrasound scanning)-MR, X-ray-MR, or the like, or any combinationthereof. In some embodiments, the subject to be scanned by the imagingsystem may be an organ, a texture, a lesion, a tumor, a substance, orthe like, or any combination thereof. Merely by way for example, thesubject may include a head, a breast, a lung, a rib, a vertebra, atrachea, a pleura, a mediastinum, an abdomen, a long intestine, a smallintestine, a bladder, a gallbladder, a triple warmer, a pelvic cavity, abackbone, extremities, a skeleton, a blood vessel, or the like, or anycombination thereof. As another example, the subject may include aphysical model. It is understood that this is not intended to limit thescope of the present disclosure. For persons having ordinary skills inthe art, a certain amount of variations, changes and/or modificationsmay be deducted under the guidance of the present disclosure. Thosevariations, changes and/or modifications do not depart from the scope ofthe present disclosure.

FIG. 1 is a schematic diagram illustrating an exemplary cryostat systemof an imaging system according to some embodiments of the presentdisclosure. In some embodiments, the cryostat system 100 may be used tocool a superconducting coil 200 of the imaging system. The imagingsystem may include a scanner, a processing device, a storage device, anetwork, etc. (not shown in FIG. 1). The scanner may include a cryostatsystem (e.g., the cryostat system 100), a superconducting coil (e.g.,superconducting coil 200), a radiofrequency (RF) coil assembly (notshown in the figure), a gradient coil assembly (not shown in thefigure), or the like, or any combination thereof.

The superconducting coil 200 of the imaging system may generate a firstmagnetic field (also referred to herein as a main magnetic field) forpolarizing a subject to be scanned. The main field may be named to B₀herein. The RF coil assembly may include a plurality of coils (e.g.,transmit coils, receiver coils, etc.) for transmitting and/or receivingRF signals. The gradient coil assembly may generate a second magneticfield (also referred to herein as a gradient magnetic field). Thegradient coil assembly may be assembled in an area surrounding ascanning region of the imaging system. The processing device may receivesignals generated by the coils (e.g., the superconducting coil, the RFcoil, and the gradient coil) and further transform the signals intoother types of data (e.g., image data). The superconducting coil 200 maywork in conjunction with the gradient coil assembly. For example, whenthe imaging system collects sequences data, the main magnetic field maybe temporarily pulsed, and the imaging system may generate sequence datafor controlling magnetic gradients of the gradient coil. Thesuperconducting coil 200 may be operative at a low temperature (e.g.,4.2 K). Therefore, the imaging system may consume a certain amount ofcold head cooling capacity (e.g., 200 MW).

The cryostat system 100 may maintain the superconducting coil 200 in asuperconducting state so that the superconducting coil 200 may workproperly. The cryostat system 100 may liquefy a vapor-state coolingmedium (e.g., vapor-state helium) into a liquid state. Thesuperconducting state refers to a state of a superconductor material inwhich the superconductor material has superconducting properties, suchas a zero electrical resistive state. In some embodiments, thesuperconductor material may be in a superconducting state when thesuperconductor material is exposed in a low-temperature ambient (e.g.,4.2 K).

As shown in FIG. 1, the cryostat system 100 may include a cold headassembly 110, a tank 120, a radiation shield 130, a vacuum layer 140,and a heat conductive band 150, etc.

The tank 120 may contain a cavity 121 to accommodate the cooling mediumand the superconducting coil 200. The cooling medium may include acoolant in a vapor state, liquid state, or both vapor and liquid states.For example, the cooling medium may include the vapor-state helium andliquid-state helium. In some embodiments, the superconducting coil 200may be immersed in the cooling medium including the vapor-state coolingmedium and the liquid-state cooling medium.

The cold head assembly 110 may be configured to cool the cooling mediumto maintain the superconducting coil 200 in a superconducting state(e.g., a low-temperature superconducting state). The cold head assembly110 may provide cooling compensation to the cooling medium since theheat from outside ambient of the imaging system may leak into the cavity121. The amount of the cooling compensation may be greater than theamount of the heat leaked into the cryostat system 100. The cryostatsystem 100 may be at a stable temperature to ensure that thesuperconducting coil 200 can work properly. Take the cooling mediumbeing helium as an example; the liquid-state helium may be stored in thecavity 121 of the tank 120. The liquid-state helium may evaporate into avapor state after absorbing the heat leaked in from the ambient. Thevapor-state helium may rise to the top of the cavity 121 and becomeclose to the cold head assembly 110. The cold head assembly 110 mayprovide a cooling capacity to the cavity 121 for exchanging heat withthe vapor-state helium. The vapor-state helium may be liquefied into aliquid state. The liquefied helium may transfer a certain amount ofcooling capacity to the bottom region of the cavity 121. The evaporationportion of the cooling medium may be reduced, thereby reducing the lossof the cooling medium. In some embodiments, the cold head assembly 110may include a cold head assembly of a GM (Gifford-Mcmahon) refrigerator,a cold head assembly of a Sterling refrigerator, or the like, or anycombination thereof.

The cold head assembly 110 may be mounted on the tank 120 (e.g., on theside of the tank 120 as shown in FIG. 1). The central axis of the coldhead assembly 110 and a vertical line may form an angle that is greaterthan 20 degrees (e.g., 25 degrees, 29 degrees, 36 degrees, 45 degrees,etc.). The central axis of the cold head assembly 110 refers to a linethat passes both the center of the top surface of the cold head assembly110 and the center of the bottom surface of the cold head assembly 110.The vertical line may pass a reference point on the central axis of thecold head assembly 110 and be parallel to a vertical plane including anaxis of the superconducting coil 200 and a cross-section of thesuperconducting coil 200. The reference point on the central axis of thecold head assembly 110 may be the center of the bottom surface of thecold head assembly 110. In some embodiments, the center of the bottomsurface of the cold head assembly 110 may be the same as an intersectionpoint of the cold head assembly 110 and the tank 120. The cold headassembly 110 may rotate with the reference point. For illustrationpurposes, a coordinate system (e.g., a Cartesian coordinate system) thatdesignates the center of the superconducting coil 200 as the origin maybe presented in FIG. 1. The coordinate system may include X axis, Yaxis, and Z axis. The Z-axis may be perpendicular to the X-axis and theY-axis. The X-Y plane may be parallel to a ground plane that thecryostat system 100 is located in (e.g., the horizontal plane). Thevertical line refers to a line that passes the reference point of thecentral axis of the cold head assembly 110 and is parallel to theZ-axis. The corresponding vertical plane including an axis of thesuperconducting coil 200 (e.g., the Z-axis and/or the Y-axis) and across-section thereof is the Z-Y plane.

In some embodiments, the cold head assembly 110 may be inclined relativeto the reference point that the top of the cold head assembly 110 may bedistant to the vertical line. The cold head assembly 110 may be inclinedin a plane parallel with the Z-X plane, inclined in a plane parallelwith the Z-Y plane or inclined in a plane not parallel with both the Z-Xplane and the Z-Y plane. The angle between the vertical line and thecentral axis of the cold head assembly 110 may range from 20 degrees to45 degrees. For example, when the angle between the vertical line andthe central axis of the cold head assembly 110 is 25 degrees, atrajectory of the central axis of the cold head assembly 110 may be acone including the vertical line as a rotation axis and the conningangle may be 50 degrees. As another example, when the angle between thevertical line and the central axis of the cold head assembly 110 is 45degrees, a trajectory of the central axis of the cold head assembly 110may be a cone including the vertical line as a rotation axis and theconing angle may be 90 degrees. In some embodiments, the cold headassembly 110 may include a first cold head 111 (shown in FIG. 2), asecond cold head 112, etc. Detailed descriptions of the cold headassembly 110 may be found elsewhere of the present disclosure (e.g.,FIG. 2, FIG. 3).

The radiation shield 130 and the vacuum layer 140 may enclose the tank120 as shown in FIG. 1. The radiation shield 130 and the vacuum layer140 may decrease the heat radiation and convection from the outsideambient of the cryostat system 100 and reduce the evaporation of thecooling medium (e.g., helium).

The heat conductive band 150 may be located in the vacuum layer 140 andmay thermal connect the radiation shield 130 and the first cold head111. The heat conductive band 150 may transmit the cooling capacity ofthe first cold head 111 to the radiation shield 130 to cool theradiation shield 130 to proper low temperature. In some embodiments, theheat conductive band 150 may be a flexible thermal conductivity band orbraid. The heat conductive band 150 may be made of a high thermalconductivity material, such as high purity copper, high purity aluminum,etc.

These descriptions are intended to be illustrative, and not to limit thescope of the present disclosure. Many alternatives, modifications, andvariations will be apparent to those skilled in the art. The features,structures, methods, and other characteristics of the exemplaryembodiments described herein may be combined in various ways to obtainadditional and/or alternative exemplary embodiments. For example, thecryostat system 100 may be a part of the imaging system. As anotherexample, the cold head assembly 110 may be mounted on the right side ofthe tank 120.

FIG. 2 is a schematic diagram illustrating an exemplary cold headposition of the cryostat system 100 in hospital field according to someembodiments of the present disclosure. In some embodiments, FIG. 2 is aside view of the cryostat system shown in FIG. 1. The cryostat system100 may include the cold head assembly 110 (including a first cold head111, etc.), the tank 120, the radiation shield 130, the vacuum layer140, and other components shown in other figures of the presentdisclosure (e.g., FIG. 1 and the descriptions thereof). For illustrationpurposes, FIG. 2 may also include a ceiling 300 and a ground plane 400indicating an operable state environment of the cryostat system 100, anda side view of the coordinate system (whose origin may be the center ofthe superconducting coil 200 as shown in FIG. 1). Therefore, FIG. 2 maybe a front view of the cryostat system 100 corresponding to the Y-Zplane. In some embodiments, the ceiling 300 and/or the ground plane 400may be parallel to the X-Y plane. The vertical line 203 that passes thereference point 205 may be perpendicular to the ceiling 300 and/or theground plane 400, and also parallel to the Z-axis.

As shown in FIG. 2, the angle between the central axis 207 of the coldhead assembly 110 and the vertical line 203 is denoted as θ. The anglemay represent an inclined degree of the cold head assembly 110 relativeto the cryostat system 100. The angle θ may be greater than 20 degreesand not greater than 45 degrees. In some embodiments, the cold headassembly may be inclined towards any direction around the referencepoint 205. For example, the cold head assembly 110 may be inclined alongan axis of the coordinate system (whose origin may be the center of thesuperconducting coil 200). For instance, the cold head assembly 110 maybe inclined along the Y-axis as shown in FIG. 2. Similarly, the coldhead assembly may be inclined around the reference point 205 along theX-axis, or any other direction in the X-Y plane.

During an installation or removal operation to the cold head assembly110, the cold head assembly 110 may need to be raised (or pulled). Thus,the space between the top of the cold head assembly 110 and the ceiling300 may be reduced during the installation or removal of the cold headassembly 110. The height of the ceiling 300 may need to be greater thanan appropriate value to ensure the operation of installation or removaloperation of the cold head assembly 110. In the present disclosure, theinclined mounting of the cold head assembly 110 may decrease the heightrequirement of the ceiling 300, which may make the operation ofinstallation or removal of the cold head assembly 110 easier.

These descriptions are intended to be illustrative, and not to limit thescope of the present disclosure. Many alternatives, modifications, andvariations will be apparent to those skilled in the art. The features,structures, methods, and other characteristics of the exemplaryembodiments described herein may be combined in various ways to obtainadditional and/or alternative exemplary embodiments. For example, theceiling 300 is unnecessarily parallel to the ground plane 400. Thevertical line 203 may be perpendicular to the ground plane 400.

FIG. 3 is a schematic diagram illustrating an exemplary cold headassembly according to some embodiments of the present disclosure. Asshown in FIG. 3, the cold head assembly 110 may include the first coldhead 111, the second cold head 112, and other components. The cold headassembly 110 may provide the cooling capacity in two stages by the firstcold head 111 and the second cold head 112, respectively. In someembodiments, some of the vapor-state cooling medium (e.g., vapor-statehelium) may become close to the second cold head 112 and be liquefiedinto the liquid state by absorbing the cooling capacity provided by thesecond cold head 112. The liquefied cooling medium (e.g., liquefiedhelium) may drop down toward the bottom region of the tank 120, whichincludes a cold head sleeve and a liquid helium vessel in an exemplarycryostat system.

The first cold head 111 may be connected with the radiation shield 130via the heat conductive band 150 (not shown in FIG. 3). In someembodiments, the first cold head 111 may be maintained at a firsttemperature, for example, 40 K. The heat conductive band 150 may conductthe cooling capacity absorbed from the first cold head 111 to theradiation shield 130. In some embodiments, the temperature of theradiation shield 130 may be reduced to more than 50 K. Therefore, theheat from outside ambient may be interrupted.

The second cold head 112 may have a taper shape. For example, the secondcold head 112 may have a truncated cone shape (as shown in FIG. 3) or atriangular prism or polygonous prism (e.g, rectangular prism) shape (notshown in FIG. 3) with a first end surface and a second end surface. Thefirst end surface may be close to the first cold head 111. The secondend surface may be away from to the first cold head 111 compared to thefirst end surface. A diameter of the first circular end surface thatclose to may be greater than a diameter of the second end surface. Insome embodiments, when the second cold head 112 s shaped as truncatedcone shape, the first end surface and the second end surface thereof maybe configured as a first circular end and a second circular end,respectively. The first circular end may be close to the first cold head111. The second circular end may be distant to the first cold head 111compared to the first circular end. The diameter of the first circularend (also referred to herein as a first diameter) may be greater thanthe diameter of the second circular end (also referred as a seconddiameter). For example, the first diameter may be 8 cm, and the seconddiameter may be 6.5 cm. In some embodiments, the second cold head 112may be maintained at a second temperature, for example, 4.2 K or lowerthan 4.2 K (4.2 K is a critical temperature of liquid-state helium). Thesecond cold head 112 may provide the cooling capacity to liquefy thevapor-state cooling medium into the liquid state when some of thevapor-state cooling medium becomes close to the second cold 112 head.The liquefied cooling medium (e.g., liquefied helium) may be depositedon a side surface of the second cold head 112, and slide down along thesurface of the second cold head 112, and further drop down toward to thebottom region of the tank 120.

These descriptions are intended to be illustrative, and not to limit thescope of the present disclosure. Many alternatives, modifications, andvariations will be apparent to those skilled in the art. The features,structures, methods, and other characteristics of the exemplaryembodiments described herein may be combined in various ways to obtainadditional and/or alternative exemplary embodiments. For example, thecold head assembly 110 may provide the cooling capacity in three stages.The cold head assembly 110 may further include a third cold head betweenthe first cold head 111 and the second cold head 112.

FIG. 4 is a schematic diagram illustrating an exemplary second cold headaccording to some embodiments of the present disclosure. As shown inFIG. 4, the second cold head 112 may include a first circular end 401, asecond circular end 403, and a side surface 405 shown in other figuresof the present disclosure (e.g., FIG. 3). The first circular end 401 maybe close to the first cold head 111. The second circular end 403 may bedistant to the first cold head 111 compared to the first circular end.The side surface 405 may include a plurality of protrusions 113. Twoadjacent protrusions of the plurality of protrusions may form a recess409.

In some embodiments, each of the plurality of protrusions 113 mayinclude a fin-like shape. Each of the plurality of protrusions 113 mayhave a first end 405 close to the first circular end 401 and a secondend 407 close to the second circular end 403. That is, each of pluralityof protrusions 113 protrusion may have the first end 405 close to thefirst cold head 111 and the second end 407 away from the first cold head111. The thickness of the first end 405 of the fin-like shape protrusion113 (also referred to herein as the first thickness) may be greater thanthe thickness of the second end 407 of the fin-like shape protrusion 113(also referred to herein as the second thickness). The first thicknessrefers to a contact region having a width between the first end 405 andthe second cold head 112. The second thickness refers to a contactregion having a width between the second end 407 and the second coldhead 112. Detailed descriptions of the second cold head 112 may be foundelsewhere of the present disclosure (e.g., the description of FIGS. 4,5, and 6).

The fin-like shape protrusions 113 may increase a contact surface areabetween the second cold head 112 and the vapor-state cooling medium(e.g., vapor-state helium) and thus increase the heat exchange surfacebetween them. For illustration purposes, in FIG. 4, the vapor-statecooling medium is denoted by wavy lines 411, and the liquid-statecooling medium is denoted by circles 413. The second cold head 112 mayexchange heat with the vapor-state cooling medium. Some of thevapor-state cooling medium may be liquefied into the liquid state. Theliquefied cooling medium may be deposited on the surface of the secondcold head 112 and flow along surfaces of the plurality of protrusions113 toward the bottom region of the tank 120 as the result of gravity.

In some embodiments, the second cold head 112 and/or the protrusions 113may be made of a high thermal conductivity material, such as high puritycopper, high purity aluminum, etc. In some embodiments, the surface ofthe second cold head 112 and/or the surface of the plurality ofprotrusions may be polished and/or have a plating layer. The platinglayer may include electroplated Cu plated by an electro-copperingtechnique, which may smooth surfaces and/or reduce frictions.

These descriptions are intended to be illustrative, and not to limit thescope of the present disclosure. Many alternatives, modifications, andvariations will be apparent to those skilled in the art. The features,structures, methods, and other characteristics of the exemplaryembodiments described herein may be combined in various ways to obtainadditional and/or alternative exemplary embodiments. For example, theone of the plurality of protrusions may include a columnar shape. Thecolumnar shape protrusion may have a first end close to the firstcircular end 401 of the second cold head 112 and a first end close tothe second circular end 403 of the second cold head 112. The thicknessof the first end of the columnar shape protrusion may be equal to thethickness of the second end of the columnar shape protrusion.

FIG. 5 is a schematic diagram illustrating an exemplary second cold headaccording to some embodiments of the present disclosure. FIG. 5 is a topview of the second cold head 112 from the perspective of the firstcircular end as arrow B shown in FIG. 4. The second cold head 112 mayinclude the plurality of fin-like shape protrusions 113. Two adjacentprotrusions of the plurality of protrusions 113 may form a recess 409 asdescribed elsewhere in the present disclosure (e.g., FIG. 4 and thedescriptions thereof).

FIG. 6 is a schematic diagram illustrating a part of an exemplary secondcold head according to some embodiments of the present disclosure. FIG.6 is an enlarged view of part C of the second cold head 112 shown inFIG. 5. The second cold head 112 may include a plurality of fin-likeshape protrusions 113 (also referred to herein as fins 113). Twoadjacent protrusions of the plurality of protrusions 113 may form arecess 409 as described elsewhere in the present disclosure (e.g., FIG.4 and the descriptions thereof). The liquefied cooling medium (denotedby circles 413 as shown in FIG. 6) may be deposited on the surface ofthe second cold head 112, and flow along the surface of the fins 113 ofthe second cold head 112 (e.g., the recesses 409).

FIG. 7 is a schematic diagram illustrating an exemplary second cold headaccording to some embodiments of the present disclosure. As shown inFIG. 6, the second cold head 112 may be shaped as a truncated coneincluding a first circular end 401, a second circular end 403, and aside surface 405 as described in FIG. 3. The side surface 405 and thehorizon surface 701 may form an angle denoted by β. In some embodiments,β may be greater than 0 degree and less than 90 degrees.

In some embodiments, the liquid helium (an exemplary cooling medium) mayhave properties including a viscosity of 3.244×10⁻⁶ Pa·s and a surfacetension coefficient of 8.954×10⁻³ N/m, at 4.2 K, 0.1 MPa, and a thermalconductivity of 0.0186 W/(m.K). When the liquefied cooling medium flowsalong the plurality of protrusions 113 of the second cold head 112, theliquefied cooling medium may form a cooling medium film. The coolingfilm may increase the thermal resistance between the second cold head112 and the surrounding vapor-state cooling medium under the affectionof a resultant force consist of an external friction, a viscous force,and a surface tension of the cooling medium. The resultant force isdenoted by f. The resultant force may include a vertical force denotedby f′. For example, f′ may be equal to f multiplied by sin β, i.e.,f′=f×sin β. The vertical force f′ may prevent the liquid-state coolingmedium from dropping to the bottom region of the tank 120.

The gravity of the liquid-state cooling medium denoted by G may begreater than vertical force denoted by f′. The tendency of the coolingmedium film to flow downward may be strengthened and the thickness ofthe cooling medium film may be reduced. Therefore, the thermalresistance between the second cold head 112 and the surroundingvapor-state cooling medium may be reduced. The heat exchange efficiencybetween the second cold head 112 and the vapor-state cooling medium maybe promoted.

Having thus described the basic concepts, it may be rather apparent tothose skilled in the art after reading this detailed disclosure that theforegoing detailed disclosure is intended to be presented by way ofexample only and is not limiting. Various alterations, improvements, andmodifications may occur and are intended to those skilled in the art,though not expressly stated herein. These alterations, improvements, andmodifications are intended to be suggested by this disclosure, and arewithin the spirit and scope of the exemplary embodiments of thisdisclosure.

Moreover, certain terminology has been used to describe embodiments ofthe present disclosure. For example, the terms “one embodiment,” “anembodiment,” and/or “some embodiments” mean that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the present disclosure.

Further, it will be appreciated by one skilled in the art, aspects ofthe present disclosure may be illustrated and described herein in any ofa number of patentable classes or context including any new and usefulprocess, machine, manufacture, or composition of matter, or any new anduseful improvement thereof. Accordingly, aspects of the presentdisclosure may be implemented entirely hardware, entirely software(including firmware, resident software, micro-code, etc.) or combiningsoftware and hardware implementation that may all generally be referredto herein as a “unit,” “module,” or “system.” Furthermore, aspects ofthe present disclosure may take the form of a computer program productembodied in one or more computer-readable media having computer readableprogram code embodied thereon.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including electro-magnetic, optical, or thelike, or any suitable combination thereof. A computer readable signalmedium may be any computer readable medium that is not a computerreadable storage medium and that may communicate, propagate, ortransport a program for use by or in connection with an instructionexecution system, apparatus, or device. Program code embodied on acomputer readable signal medium may be transmitted using any appropriatemedium, including wireless, wireline, optical fiber cable, RF, or thelike, or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of thepresent disclosure may be written in any combination of one or moreprogramming languages, including an object-oriented programming languagesuch as Java, Scala, Smalltalk, Eiffel, JADE, Emerald, C++, C#, VB. NET,Python or the like, conventional procedural programming languages, suchas the “C” programming language, Visual Basic, Fortran 2103, Perl, COBOL2102, PHP, ABAP, dynamic programming languages such as Python, Ruby, andGroovy, or other programming languages. The program code may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider) or in a cloud computing environment or offered as aservice such as a Software as a Service (SaaS).

Furthermore, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations, therefore, is notintended to limit the claimed processes and methods to any order exceptas may be specified in the claims. Although the above disclosurediscusses through various examples what is currently considered to be avariety of useful embodiments of the disclosure, it is to be understoodthat such detail is solely for that purpose and that the appended claimsare not limited to the disclosed embodiments, but, on the contrary, areintended to cover modifications and equivalent arrangements that arewithin the spirit and scope of the disclosed embodiments. For example,although the implementation of various components described above may beembodied in a hardware device, it may also be implemented as asoftware-only solution, for example, an installation on an existingserver or mobile device.

Similarly, it should be appreciated that in the foregoing description ofembodiments of the present disclosure, various features are sometimesgrouped together in a single embodiment, figure, or description thereoffor the purpose of streamlining the disclosure aiding in theunderstanding of one or more of the various inventive embodiments. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed subject matter requires more features thanare expressly recited in each claim. Rather, inventive embodiments liein less than all features of a single foregoing disclosed embodiment.

In some embodiments, the numbers expressing quantities or propertiesused to describe and claim certain embodiments of the application are tobe understood as being modified in some instances by the term “about,”“approximate,” or “substantially.” For example, “about,” “approximate,”or “substantially” may indicate ±20% variation of the value itdescribes, unless otherwise stated. Accordingly, in some embodiments,the numerical parameters set forth in the written description andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by a particular embodiment. Insome embodiments, the numerical parameters should be construed in lightof the number of reported significant digits and by applying ordinaryrounding techniques. Notwithstanding that the numerical ranges andparameters setting forth the broad scope of some embodiments of theapplication are approximations, the numerical values set forth in thespecific examples are reported as precisely as practicable.

Each of the patents, patent applications, publications of patentapplications, and other material, such as articles, books,specifications, publications, documents, things, and/or the like,referenced herein is hereby incorporated herein by this reference in itsentirety for all purposes, excepting any prosecution file historyassociated with same, any of same that is inconsistent with or inconflict with the present document, or any of same that may have alimiting affect as to the broadest scope of the claims now or laterassociated with the present document. By way of example, should there beany inconsistency or conflict between the description, definition,and/or the use of a term associated with any of the incorporatedmaterial and that associated with the present document, the description,definition, and/or the use of the term in the present document shallprevail.

In closing, it is to be understood that the embodiments of theapplication disclosed herein are illustrative of the principles of theembodiments of the application. Other modifications that may be employedmay be within the scope of the application. Thus, by way of example, butnot of limitation, alternative configurations of the embodiments of theapplication may be utilized in accordance with the teachings herein.Accordingly, embodiments of the present application are not limited tothat precisely as shown and described.

What is claimed is:
 1. A cryostat for a magnetic resonance imagingsystem, comprising: a tank containing a cavity to accommodate a coolingmedium and a superconducting coil; and a cold head assembly configuredto cool the cooling medium to maintain the superconducting coil in asuperconducting state, the cold head assembly being mounted on the tank,a central axis of the cold head assembly and a vertical line forming anangle, the vertical line passing a reference point of the central axisof the cold head assembly, the vertical line being parallel to avertical plane including an axis of the superconducting coil and across-section of the superconducting coil, the cold head assemblyincluding a first cold head and a second cold head; and the second coldhead having a taper shape with a first end surface close to the firstcold head and a second end surface away from the first cold head.
 2. Thesystem of claim 1, wherein the cold head assembly is inclined awayrelative to the vertical plane.
 3. The system of claim 1, wherein thecold head assembly is inclined along the axis of the superconductingcoil.
 4. The system of claim 1, wherein the angle is equal to or lessthan 45 degrees.
 5. The system of claim 1, wherein a surface of thesecond cold head includes a plurality of protrusions, a recess beingformed between two adjacent protrusions of the plurality of protrusions.6. The system of claim 5, wherein: one of the plurality of protrusionshas a fin-like shape; the one of the plurality of protrusions has afirst end close to the first cold head and a second end away from thefirst cold head; and a thickness of the first end of the one of theplurality of protrusions is greater than a thickness of the second endof the one of the plurality of protrusions.
 7. The system of claim 5,wherein the surface of the second cold head or a surface of theplurality of protrusions is polished or has a plating layer.
 8. Thesystem of claim 7, further comprising: a radiation shield; a vacuumlayer, the radiation shield and the vacuum layer enclosing the tank; anda heat conductive band located in the vacuum layer and configured tothermal connect the radiation shield and the cold head assembly.
 9. Thesystem of claim 1, wherein the taper shape includes a shape oftriangular prism, polygonous prism or truncated cone.
 10. The system ofclaim 1, wherein a diameter of the first end surface being greater thana diameter of the second end surface.
 11. A system, comprising: a coldhead assembly including at least a first cold head and a second coldhead, wherein the second cold head has a taper shape with a first endsurface close to the first cold head and a second end surface away fromthe first cold head; and a diameter of the first circular end is greaterthan a diameter of the second circular end.
 12. The system of claim 11,wherein a surface of the second cold head includes a plurality ofprotrusions, a recess being formed between two adjacent protrusions ofthe plurality of protrusions.
 13. The system of claim 12, wherein one ofthe plurality of protrusions has a fin-like shape; the one of theplurality of protrusions has a first end close to the first cold headand a second end away from the first cold head; and a thickness of thefirst end of the one of the plurality of protrusions is greater than athickness of the second end of the one of the plurality of protrusions.14. The system of claim 13, wherein the surface of the second cold heador a surface of the plurality of protrusions is polished or has aplating layer.
 15. The system of claim 11, further comprising a tank,containing a cavity to accommodate a cooling medium and asuperconducting coil, wherein the cold head assembly is mounted on aside of the tank and configured to cool the cooling medium to maintainthe superconducting coil in a superconducting state, a central axis ofthe cold head assembly and a vertical line forming an angle, thevertical line passing a reference point of the central axis of the coldhead assembly, the vertical line being parallel to a vertical planeincluding an axis of the superconducting coil and a cross-section of thesuperconducting coil.
 16. The system of claim 15, wherein the cold headassembly is inclined away relative to the vertical plane.
 17. The systemof claim 15, wherein the cold head assembly is inclined along the axisof the superconducting coil.
 18. The system of claim 15, wherein theangle is equal to or less than 45 degrees.
 19. The system of claim 15,further comprising: a radiation shield; a vacuum layer, the radiationshield and the vacuum layer enclosing the tank; and a heat conductiveband located in the vacuum layer and configured to thermal connect theradiation shield and the cold head assembly.
 20. The system of claim 11,wherein the taper shape includes a shape of a triangular a prism, apolygonous prism, or a truncated cone.